As anyone who has tended a garden knows, even relatively minor changes in
climate and temperature can have a pronounced effect on plant life. Less
understood, perhaps, is that en masse plants can in turn have a sizable impact
on the climate. Regionally, an increase in plant growth can cool surface
temperatures and give rise to more rain and cloud cover. On a global scale, a
rise in plant growth can lower the amount of atmospheric carbon dioxide, which
is one of the most abundant of the greenhouse gases, and potentially cool the
atmosphere.

For many years biologists and Earth scientists have known of these
interactions, but they have never been able to assess to what degree plants
influence climate. The problem has always been how to observe this influence. In
order to understand the interaction of vegetation with the climate, researchers
must have long-term, worldwide measurements of plant growth. So far scientists
have met with limited success. While they have obtained qualitative records of
relative plant density across the world using satellite imagery, they still
cannot quantitatively assess the degree to which vegetation changes from one
month to the next. For years, labs around the world have been working towards
this goal.

Although it’s obvious
that climate determines what types of plants live in a given area, plants themselves
have an effect on climate. Lush vegetation absorbs sunlight, cools the Earth’s
surface and increases humidity. Current climate models don’t always account for
these effects, so scientists are developing new datasets based on satellite data to
use in improved models. (Photographs courtesy Philip Greenspun)

Remote sensing specialists at the University of Boston and NASA may now be
onto a solution. Using a measurement known as Leaf Area Index, they have found a
way to quantify plant growth on a global scale with satellite imagery. Their
method can pinpoint when leaves begin to grow in a region, when they fall off,
and how dense they become at peak growing season. With the aid of instruments
such as MODIS aboard NASA’s Terra satellite and the AVHRR instrument aboard
NOAA’s polar satellites, the researchers may soon be able to forecast the ways
in which plants impact our weather and global climate.

The two images above show the changes in Leaf Area Index (LAI)
that occured during the spring of 2002 in Switzerland and eastern France. White corresponds to an LAI of 0,
while green, dark green, and blue represents increasingly dense vegetation. Topographic shading emphasizes the
barren peaks of the Alps. (Data provided by Boston University Climate and
Vegetation Research Group. Images by Robert Simmon)